1. Field of the Invention
The present invention is directed generally toward precision electrical measurement devices and, more specifically, to printed circuit boards for measurement devices including a low-current measuring region isolated from neighboring regions of the printed circuit board.
2. Description of Related Art
In devices for measuring very low currents (e.g., nanoamperes and below), the dielectric material of printed circuit boards absorb unacceptable numbers of electrons or other charge carriers at the low current nodes of the devices. This absorption, referred to as dielectric absorption, can introduce significant error levels into the measured current due to the low magnitude of the current being measured.
Efforts to minimize dielectric absorption have typically included insulating a test terminal 1 with a dielectric 7 recessed within an uppermost layer 5 of a substrate, as shown in FIG. 1. To form such a structure, a portion of the substrate's uppermost layer 5 must first be machined away, leaving a void 9. Each of the side walls defining the void is plated with an electrical conductor 3. An electrical conductor 4 is also provided to the floor of the void 9, thereby forming a conductive liner recessed within the uppermost layer 5 of the substrate. With the liner and dielectric 7 recessed in this manner, the test terminal 1 is located approximately flush with an exposed surface 8 of the uppermost layer 5 of substrate.
Although isolating a test terminal 1 with a dielectric 7 that is embedded and separated from the substrate's uppermost layer 5 is an approach for addressing dielectric losses encountered when measuring low-currents, such a structure has several shortcomings. In order to form such a structure a portion of the substrate must first be removed to form the void 9. This weakens the mechanical strength of the substrate, and requires milling operations to be performed, adding to the complexity and cost of manufacturing such substrates. Further, such structures are not practical for insulating much more than the test terminal 1 shown in FIG. 1 due to the extensive machining that would be required, causing significant weakening of the substrate's rigidity.
Accordingly, there is a need in the art for a method and apparatus for minimizing dielectric losses experienced by circuits for conducting measurements of low currents. The method and apparatus can minimize manufacturing complexity and cost, guard against dielectric loss in desired regions of a printed circuit board, and can have sufficient mechanical strength to minimize damage to electrical components that extend between a guarded region and an adjacent region.